Behaviour and variability of local and regional oxidant levels (OX = O₃ + NO ₂) measured in a polluted area in central-southern of Iberian Peninsula

The purpose of this work is to contribute to the understanding of the photochemical air pollution in central-southern of the Iberian Peninsula, analysing the behaviour and variability of oxidant levels (OX = O₃ + NO₂), measured in a polluted area with the highest concentration of heavy industry in central Spain. A detailed air pollution database was observed from two monitoring stations. The data period used was 2008 and 2009, around 210,000 data, selected for its pollution and meteorological statistics, which are very representative of the region. Data were collected every 15 min, however hourly values were used to analyse the seasonal and daily ozone, NO, NO₂ and OX cycles. The variation of OX concentrations with NO ₓ is investigated, for the first time, in the centre of the Iberian Peninsula. The concentration of OX was calculated using the sum of a NO ₓ -independent ‘regional’ contribution (i.e. the O₃ background), and a linearly NO ₓ -dependent ‘local’ contribution. Monthly dependence of regional and local OX concentration was observed to determine when the maximum values may be expected. The variation of OX concentrations with levels of NO ₓ was also measured, in order to pinpoint the atmospheric sources of OX in the polluted areas. The ratios [NO₂]/[OX] and [NO₂]/[NO ₓ ] vs. [NO ₓ ] were analysed to find the fraction of OX in the form of NO₂, and the possible source of the local NO ₓ -dependent contribution, respectively. The progressive increase of the ratio [NO₂]/[OX] with [NO ₓ ] observed shows a greater proportion of OX in the form of NO₂ as the level of NO ₓ increases. The higher measured values in the ratio [NO₂]/[NO ₓ ] should not be attributed to NO ₓ emissions by vehicles; they could be explained by industrial emission, termolecular reactions or formaldehyde and HONO directly emitted by vehicles exhausts. We also estimate the rate of NO₂ photolysis, J NO₂ = 0.18–0.64 min⁻¹, a key atmospheric reaction that influence O₃ production and then the regional air quality. The first surface plot study of annual variation of the daily mean oxidant levels, obtained for this polluted area may be used to improve the atmospheric photochemical dynamic in this region of the Iberian Peninsula where there are undeniable air quality problems.